Time Division Multiple Access or TDMA represents one way of dividing restricted radio frequency resources between several users. In time division multiple access systems, each frequency band is divided into time slots. Each user obtains a transmission or reception turn in a specific time slot. Time division multiple access requires a pulse-like transmission, meaning that each sender sends signal in a specific time slot and ends the transmission when the time has ran out. Consequently a need arises to ramp up and to ramp down the power of the transmitter as rapidly as possible in order to use the specific transmission turn efficiently and to disturb the other users of the same frequency as little as possible. In practice, the transmission pulses spread out from the ideal rectangle shape and slide slightly over one another, thus interfering with one another at the beginning and end of the pulse. In practice a rectangle pulse is not used, as it causes interference to the frequency spectrum, wherefore safety times have been defined in the GSM system (Global System for Mobile Communication) to ramp up and ramp down the power. If the latest received pulse in the detection is weak and the previously received pulse is strong, it is obvious that the spread of the pulses causes interference to the first symbols of the weaker pulse.
The filters in the receiver as well as the radio channel spread impulse response, thus causing interference. How much impulse response the filters spread, depends on the number of taps in the filter that deviate from zero: the larger the number of taps the wider the impulse response becomes. Then again, the idea is to obtain as much as possible of the energy in the received signal to the impulse response. The longer the impulse response of the filter in the receiver is, the more of the energy in the received signal is obtained into the impulse response estimate.
Generally time division multiple access is associated for example with frequency division multiple access. An example of such a system is the GSM, in which a 200 kHz frequency band is divided into eight time slots, each one being 577 μs long. A part of the time slot is allocated to ramp up and ramp down the power, each user in the system therefore has 542,8 μs or 147 bits of effective message transmission time.
Currently a need is created to increase the data transmission rate also in wireless data transmission in order to be able to transfer data in addition to speech and to offer, for example, wireless Internet services to the end users of the telecommunications systems. Such a development has increased the requirements for the interference tolerance of the systems, since a higher data transmission rate requires a more efficient modulation method, and a more efficient modulation method results in a system that is more liable to interference.
In cellular radio networks large radio cells, or macro cells, can be formed. Then the operators may cover geographically large areas with a few base stations. The solution is advantageous in sparsely inhabited areas, where traffic is scarce and therefore only a few radio channels are needed in each cell. However, the near-far problem has to be resolved in macro cells. Such a problem is created in a situation in which the base station receives transmission from both a subscriber terminal located near the base station and from a subscriber terminal located far from the base station. A signal arriving from far attenuates along the way and a signal arriving from nearby may efficiently interfere with it. The worst situation occurs if both transmitters employ the same frequency and are placed in adjacent time slots.